AO cells find usages in laser scanning systems of high precision, "high precision" being understood to mean an accuracy of position of at least 1 part in ten thousand. Further, such systems involve a settling time when changing frequency of less than one microsecond. High speed access at a high degree of repeatability is also required. It has been found that digitally generated signals are preferred over analog methods in general.
Voltage control oscillators (VCOs) often change frequency with temperature. This is highly undesirable in many applications. Phase locked loops (PLLs) are more stable than VCOs, but cannot often achieve the required quick settling time.
The prior art includes systems of generating such frequencies using a sampled sinusoid. Means to perform such methods suffer from a high degree of complexity and a corresponding high cost. They require an accumulator, a sine look-up table and a digital to analog converter. Often each of these components must be clocked at rates in excess of double the highest frequency to be generated. That is a severe disadvantage which is overcome by the invention. This disadvantage, and the corresponding advantage of the invention, is further multiplied in that the components are often clocked at three to ten times the highest frequency to be generated. For example, if a 100 MHz sinusoid is desired, the clock to drive the system will often function at a minimum of 200 MHz and will more likely be operated at a frequency in excess of 300 MHz.
Thus, while the sampled sinusoid method and apparatus can achieve acceptable accuracy, spectral purity, and fast settling, it has a severe disadvantage of being complicated and relatively expensive.